The present invention relates to the field of using brazing to assemble together a titanium metal piece and a piece made of a ceramic material based on silicon carbide (SiC) and/or carbon.
In known manner, ceramic materials are characterized by their mechanical properties that make them suitable for constituting structural elements, and by their ability to conserve these mechanical properties at high temperature. Such materials are used in particular for fabricating pieces that are subjected to high thermomechanical stresses in aviation applications (engine parts or fairing elements).
Ceramic materials and metals are traditionally assembled together by a mechanical connection of the riveting or bolting type, which connection is sometimes unsuitable because of reasons of size or difficulties of implementation.
Furthermore, known methods of uniformly assembling ceramic materials that make use of organic precursors of ceramics are not adapted to heterogeneous assemblies between a ceramic material and a metal.
Furthermore, known brazing technologies used for making homogeneous ceramic/ceramic bonds are difficult to use for heterogeneous brazing between a ceramic material and a metal because of the very different thermomechanical and chemical behaviors of ceramic materials and of metals.
More precisely, if it is desired to use brazing to assemble a ceramic material on a metal alloy based on titanium, aluminum, and vanadium, then the assembly is confronted with a very large difference in expansion between those two items, given that the coefficient of expansion of such a metal alloy is about two to five times greater than the coefficient of expansion of ceramic materials. As a result, for a typical 30 millimeter (mm) assembly, it is necessary to accommodate an expansion offset of 0.2 mm on cooling the assembly from the solidification temperature of the brazing composition to ambient temperature.
The large amount of relative shrinking of the metal piece leads to high levels of stress between the two pieces, and in particular to a compression zone in the brazed joint adjacent to the ceramic piece and a traction zone adjacent to the metal piece.
As a result, the assembly bends, giving rise to stresses that can lead to rupture in one of the components, and to a brazed joint of poor strength because of its localized deformation.
The invention proposes solving the above-mentioned problem by interposing, between the metal piece and the ceramic material piece, intermediate pieces having coefficients of expansion that vary progressively so as to constitute a stack of elements that are to be assembled together thermally in pairs by brazing.
This gives rise to a problem of selecting the intermediate materials, and brazing compositions that are compatible with those materials, which compositions need to satisfy general problems in terms of chemical compatibility, and in particular they must provide a so-called “chemical barrier” function making it possible firstly to avoid elements migrating from the ceramic material (silicon carbide, carbon, . . . ) to the metal piece, or vice versa, and secondly to prevent undesirable chemical compounds forming.
More precisely, chemical and thermomechanical incompatibility between the ceramic material piece and the metal prevents direct brazing being performed for high temperature operation because:                most materials react very strongly with SiC above 1000° C., leading to numerous pores and fragile intermetallic compounds with low melting points being formed, which is very damaging for the mechanical strength of such assemblies; and        the very great difference between the coefficients of thermal expansion (CTE) of metals (10 to 20×10−6 per degree Celsius) and SiC-CMCs (2 to 6×10−6 per degree Celsius) gives rise to high levels of residual stress at the interfaces leading to the assembly rupturing on cooling.        
This incompatibility constitutes a major difficulty requiring the chemical, geometrical, and method aspects to be investigated simultaneously.